This disclosure relates in general to an integral compressor-expander assembly used in refrigeration applications, particularly, in the liquefaction of natural gas. The term “integral” is generally defined to mean that the compressor and expander are mounted on a single, common shaft. In such refrigeration applications, cold gas expanders are used to produce low temperatures via pressure reduction of a flowing gas. Typically, mechanical energy is recovered from the expander to serve a useful purpose. The mechanical energy recovered from the expander can be provided to an electric generator, or to a compressor adapted to compress a gas stream. Recovering the energy directly to compression is usually the most efficient and cost effective means as it eliminates costly power generation equipment and associated energy losses.
Typically, when energy is recovered to compression, the configuration of the assembly is a single expander with radial inflow and axial outflow overhung on one end of a central shaft, and a single-stage compressor with axial inflow and radial outflow overhung on the other end of the shaft. In this configuration, the compression duty is constrained to precisely match the expansion duty to keep the assembly in power balance with no external driver or load. In addition, the pressure rise in the compressor is constrained to the amount that can be achieved by a single impeller.
When the expansion duty exceeds the compression duty or vice versa, an integrally geared arrangement may be used. With the integrally geared arrangement, multiple compressors with axial inlets and radial discharges may be driven by a single gear on multiple shafts, with the expander driving this same gear from another shaft. The gear may also be coupled to an external driver to provide additional power in the event the compression duty exceeds the expansion duty. The integral gear arrangement requires several bearings and seals, making its design complicated and leading to lower reliability and higher frequencies of machine downtime.
Therefore, there is a need to facilitate efficient energy recovery while achieving higher compression ratio in a single, reliable assembly.